ITU-T Recommendation G.993.5 entitled “Self-FEXT Cancellation (Vectoring) for Use with VDSL2 Transceivers” describes vectoring which is a crosstalk cancellation mechanism for DSL lines. Crosstalk induced by disturbing DSL lines into a victim DSL line is cancelled by adding an anti-signal to the victim DSL line that compensates for the crosstalk noise from the disturbing DSL lines. Vectoring can be done for plural disturbing DSL lines enabling to suppress simultaneously the crosstalk from all these disturbing DSL lines within the victim DSL line. In the downstream direction, i.e. the direction from the central office (CO) to the customer premises equipment (CPE), crosstalk cancelling is done by a pre-coder that pre-compensates for crosstalk by adding the anti-signal of the crosstalk to the wanted DSL signal that will be transmitted over the victim DSL line. In the upstream direction, i.e. the direction from CPE to CO, crosstalk cancellation is executed by a post-coder that post-compensates for crosstalk by adding the anti-signal of the crosstalk to the DSL signal received from the victim DSL line.
Crosstalk cancellation relies on grouping all communication lines that interact with each other, e.g. the lines within the same binder or cable bundle, in a so called vectoring group. In such vectoring group, each communication line is considered as a disturbing communication line inducing crosstalk in all other lines (and named “disturber” throughout the remainder of this patent application), and each communication line is considered as a victim communication line receiving crosstalk from all other lines (and named “victim” throughout the remainder of this patent application). The induced crosstalk from one disturber into one victim is separated in two parts: a transfer function or so called “crosstalk channel”, i.e. a function between the two lines describing the crosstalk coupling, and the transmit Power Spectral Density (PSD) of the signal transmitted on the disturber. There is a particular crosstalk channel for any combination of a disturber and a victim. The induced crosstalk is calculated as the multiplication of the particular crosstalk channel and the transmit PSD of the disturber. The crosstalk channel between a disturber and a victim is measured for instance during initialisation and represented by a crosstalk channel coefficient in a crosstalk channel matrix wherein rows represent victims and columns represent disturbers, or vice versa. To measure for instance the crosstalk channel coefficient between two VDSL lines, a pilot signal is superimposed onto the SYNC symbols during initialisation of the disturbing VDSL line, and correlation techniques are applied to recognize the crosstalk noise induced by this pilot signal into the victim VDSL line. The crosstalk channel coefficient is then calculated from the known pilot signal and the sensed noise. If the victims represent rows and the disturbers represent columns in the crosstalk channel matrix, each row in the crosstalk channel matrix constitutes a vector representing the entire crosstalk induced by all disturbers in the vectoring group into a single victim. Theoretically, such vector enables to cancel the crosstalk of all disturbers into the victim simultaneously through addition of a single anti-signal that represents the inverse of the aggregate crosstalk.
Unfortunately, technologic restrictions prevent creating a vectoring system that can cancel the crosstalk of all lines in a large vectoring group. In order to cancel the crosstalk within a vectoring group of for instance 400 VDSL lines, in the order of 1013 Multiply and Accumulate (MAC) operations per second must be executed. This is at present technologically not feasible in a cost and power efficient way. As a consequence, partial crosstalk cancellation is implemented today, enabling to cancel crosstalk of a limited number of disturbers out of a vectoring group within a victim, e.g. at most M=16 disturbers. These 16 disturbers may be arbitrarily selected within the vectoring group but for obvious reasons preferably represent the most dominant disturbers for the victim under consideration. Typically, an algorithm will be implemented to determine the M most dominant disturbers for each victim such that the crosstalk induced by these M disturbers can be cancelled while crosstalk coming from other disturbers in the vectoring group remains un-cancelled thereby minimizing the total residual noise on the victim and therefore optimizing the bit rate achievable on the victim. This algorithm may create a single list of dominant disturbers based on the full spectrum or it may create multiple lists for arbitrary parts of the spectrum.
In case of a crosstalk noise variation at a point in time where crosstalk of the maximum number M of disturbers is already cancelled, this crosstalk noise variation may impact the stability of the victims. The crosstalk noise variation may result from an additional line starting up or an increasing disturbance from a line in the vectoring group as a result of a crosstalk channel change or an increased signal PSD transmitted by the disturber. A crosstalk channel change may for instance be caused by environmental changes such as temperature variations, wire tension changes, rain or moisture leakage, etc., as a result of which the disturber and victim lines become more strongly coupled. As a result of a crosstalk noise variation, a non-cancelled line of the vectoring group may become a more dominant disturber that needs to be cancelled. The straightforward approach in case of partial crosstalk cancellation will estimate if the new crosstalk noise resulting from the newly start-up line or the varying crosstalk channel or increased PSD transmit signal of a particular disturber is above the crosstalk noise level of the least dominant disturber out of the M cancelled disturbers for a particular victim. In case the new crosstalk noise is below the crosstalk noise level of the least dominant cancelled disturber for a particular victim, the new line will be started without cancelling its crosstalk noise into the particular victim or the particular disturber with changed crosstalk noise will remain un-cancelled for the particular victim. Unfortunately, startup of the new line or leaving the disturber with varying crosstalk channel or varying transmit PSD un-cancelled, will create a sudden crosstalk noise boost, i.e. a noise residue in any victim that can affect the stability of these victims. In case the new crosstalk noise is above the crosstalk noise level of the least dominant cancelled disturber, this least dominant disturber will no longer be cancelled. It will be removed from the list of M cancelled disturbers, and will be replaced by the new or increased disturber. Anyhow, also this situation will cause a sudden boost in crosstalk noise resulting from the least dominant disturber that is no longer cancelled. This sudden crosstalk noise boost may also cause instabilities on victim lines.
In summary, whereas full crosstalk cancellation based on vectoring is technologically not feasible for large vectoring groups, partial crosstalk cancellation suffers from instability problems in case of crosstalk noise variation.
It is an objective of the present invention to disclose a device and method for crosstalk cancellation in a vectoring group that overcomes the above mentioned drawback, enabling victim lines to cope with crosstalk noise variation without risking instabilities.